Topic: using the "Offset Field" when converting Tied Points to GCPS

Rather than selecting bands containing E/W and N/S displacement for the "Offset Field" option (which accounts for the regional-scale deformation), can I give COSI-Corr two text files with long/lat/displacement in column format corresponding to the displacement at each of the GCP's I have selected in the x and y domain?

If so, must these be in geodetic or utm coordinates? If geodetic, should the displacement column (i.e. z column) be in degrees or meters?


Re: using the "Offset Field" when converting Tied Points to GCPS

Hi James,

The "offset field" expect a two-band file, the first band being the E/W a priori displacement, the second being the N/S displacement. The exact displacement is interpolated at the GCP location internally (the reason you cannot input a text file directly).  The offset field should be of have the same format as the correlation files (UTM, WGS-84 reference frame).

If you want to only input the expected displacement at the GCP location, you can ignore the "offset field" button when you build your GCP, and then input the dx,dy,dz values manually in the gcp text file generated.



Re: using the "Offset Field" when converting Tied Points to GCPS

Thanks Sebastien... of course... I didn't think of that!

I am also wondering about the optimization process:

I have selected TP's between an orthorectified "post-event" SPOT5 image and a "pre-event" aerial photo, and converted these to GCP. I then manually changed the x,y,z GCP values to take into account the regional displacement field. So I now have a file of pre-event GCP's. Can I still optimize these GCP's using the "post-event" SPOT5 image as the reference image, and which obviously doesn't have the regional displacement correction I manually entered for the pre-GCP's? (I might not have explained that very well!...) Basically, I am worried there will be a problem during optimization as the post-event reference image will not have the displacements, which have been taken account of in the GCP's of the pre-event aerial photos.

Hmmm.... and I being stupid?


Re: using the "Offset Field" when converting Tied Points to GCPS

Hi James, I'm not sure I understand things very clearly of what your pb is, but the offset field should represent the expected ground motion at the GCP between the master and the slave images. If your master (reference) image is a post-event image, and you are optimizing the GCP from the pre-event image, then the displacement should be of opposite sign to the one given by a dislocation model (it's like a dislocation model going backward in time). Does that answer your question?